A structure including a rechargeable DC power supply, an inverter and an AC motor has been known as a typical motor drive system. This motor drive system is configured as follows. During power running of the AC motor, a power supplied from the DC power supply drives and controls the AC motor so that the power can be bidirectionally supplied and received between the DC power supply, the inverter and the AC motor. Also, during a regenerative operation of the AC motor, the inverter converts the regenerative power supplied from the AC motor into a DC voltage, which can be used for charging the DC power supply by it.
In this motor drive system, the AC motor may generate a power exceeding an acceptable power of the input side (DC power supply side) and thereby may generate a surplus power. In this case, an excessively large regenerative power may generate an overvoltage inside the motor drive system. Therefore, such a control structure has been required that increases power consumption in the AC motor when require, and thereby suppresses the generative power on the input side.
In connection with this, Japanese Patent Laying-Open No. 2005-102385 (Patent Document 1) has disclosed a control structure for reducing the regenerative power. This control structure superimposes high-frequency components on d- and q-axis current command values, and thereby increases high-frequency components of a motor current flowing through an AC motor. This structure increases a core loss of the AC motor and thereby lowers drive efficiency so that the regenerative power from the AC motor is suppressed. Consequently, it is possible in the regenerative operation of the AC motor to suppress rising of a DC voltage (DC link voltage) on the input side of the inverter.
However, Patent Document 1 has not disclosed a control structure for causing the output torque to follow the torque command value when the torque command value changes during consumption control of the regenerative power described above.
Further, in the control structure disclosed in Patent Document 1, since the high-frequency components are added to the motor current, the amount of the power consumption continuously changes when viewed on extremely short time units, and the operation of the whole control system may become instable. Therefore, the high-frequency current amount that can be superimposed, i.e., the amount of power loss in the motor can increase only to a limited extent in view of the stabilization of the AC motor control. Accordingly, it is difficult to ensure a large surplus power that can be consumed while keeping torque controllability in the AC motor.